↑ a b c d Kirk y Kay, 2004, pp. ↑ a b c Bolwig, N. (1959). «Observations and thoughts on the evolution of facial mimic». ↑ a b c d e f Jolly, 2003, pp. ↑ a b c d e f g h i Schmid y Stephenson, 2003, pp. ↑ a b c d e f g h Ankel-Simons, 2007, sudaderas psg pp.